The preferred embodiment concerns a method for operation of a data bus and a data bus system, in particular to control apparatuses to which multiple bus participants are connected with a respective serial number specific to them, for example printers, in particular electrophotographic high-capacity printers.
A typical data bus to control apparatuses is the CAN bus. Individual bus participants (that are electrical modules or assemblies) of an apparatus (for example a printing device) are connected with one another with the CAN bus. The CAN bus is a simple, cost-effective, serial data bus with which data can in fact be transferred very simply, however with a relatively low transfer rate in the environment of microprocessors. According to the CAN protocol, to transfer the data, the data can be distributed in multiple frames, wherein each frame is comprised of a data block and an identifier.
The CAN bus or the corresponding protocol is explained in detail in “CAN—Controller Area Network, Grundlagen und Praxis”, 2nd Edition, 1997 (ISBN 3-7785-2575-1). A CAN bus is typically realized with a differential two-wire conductor. A CAN data frame can comprise a start bit, an identifier comprising 11 bits, an additional 7 control bits, 0 to 8 data bytes and additional control bits that follow the data bytes. The structure of a CAN data frame can differ depending on the specification. There is a CAN specification 2.0A and a CAN specification 2.0B.
The arbitration is executed per-bit and without a controller. This means that the sender (which prevails in the data bus relative to other senders based on its priority) does not need to re-send its frame. The control bits provided by the data bits are used for arbitration, wherein the arbitration essentially occurs via the bits of the identifier. It proceeds from Etschberger, Konrad, “Identifier zuordnen und Nachrichten austauschen”, (ISBN 3-8259-1902-1), 1st edition, 1998, PRAXIS Profiline—Controller Area Network (CAN), pages 40 through 43, that the type of the identifier assignment can be designated as the basic structural element of CAN-based systems, since the identifier defines a CAN message, its relative priority and thus its latency period. Moreover, the scheme of the association of identifiers with CAN messages ultimately determines the communication structure of the network.
An apparatus-oriented assignment scheme for identifiers which enables a data exchange between a superordinate control apparatus (master) with up to 127 slave apparatuses is provided in CANopen with what is known as a “minimal apparatus configuration”. The maximum number of participants in such a CANopen network is 128. This participator number is limited by the 11-bit identifier.
The difference between the data frames with standard frames that have the 11-bit identifier and with the extended frames that have the 29-bit identifier is explained in detail in Zeltwanger, Holger, “Jeder darf Senden und alle Empfangen; CAN-Kommunikation und Bus-Arbitrierung”, (ISBN 3-8251-1902-1), 1st Edition, 1998, Praxis Profiline—Controller Area Network (CAN), pages 5-7.
A device for a line termination of 2-wire conductors, in particular for a CAN bus system, arises from WO 2004/062219. This device has a first and second termination resistors between the two wires of the data conductor, wherein the first and second termination resistors are connected in series. Switches that are activated by a control logic are provided between the two termination resistors. The control logic is connected with a microcontroller, such that the control logic can activate the switches arranged between the termination resistors after receiving a corresponding signal from the microcontroller. The configuration of the termination resistor can thus be changed at any time. Alternatively, an adaptation of the termination resistor can be provided in hardware by means of a bridge in the cable harness plug.
A branch device for a data bus (for example a CAN bus) arises from WO 02/056545 A2. With this branching device, individual branches can be decoupled or coupled in a star-shaped bus topology. This is possible during the operation of the data bus. This branching device has an interface transducer, a transmission and reception unit and one or more termination resistors.
Methods for automatic address assignment in data bus systems with multiple bus participants arise from DE-U1-299 07 265 and DE-C2-197 13 240.
A local network with a master and multiple slaves in which the network addresses are automatically allocated after the connection of a new slave is described in EP 0 493 905 A1. A preliminary network address is hereby calculated using the serial number of the respective slave. This calculation occurs at the slave. The master determines the preliminary network addresses of the slaves and assigns each slave a new network address that is shorter than the preliminary network address. In the event that a conflict should exist between two preliminary network addresses, this conflict is resolved using the serial numbers of the corresponding slaves.
An arrangement to connect processor systems by means of a data bus arises from WO 94/07318. The signal lines of the data bus require a termination resistor. The processor systems respectively have an input plug and an output plug so that they can be connected in series with another processor system by means of a cable. Termination resistors for the signal lines are provided at each processor system. They are connected by a switching device that is electrically connected with a contact in the output plug. This contact of the output plug is connected via a conductor of the connection cable with a corresponding contact of an input plug of a following processor system. In the following processor system, a predetermined voltage (in particular ground) is applied at the corresponding contact. This is detected by the switching device to switch the termination resistors of the preceding processor system. The last processor system of the processor systems connected in series has no connection to an additional processor system at the input plug. The contact of the corresponding input plug that is connected with the switching element is correspondingly not connected with the predetermined voltage potential. The switching element hereby connects the termination resistors to the corresponding signal lines, whereby these are terminated.
Apparatuses (for example high-capacity printers) have a modular design. It should thus be possible to add or remove individual modules or assemblies as simply as possible. These modules and assemblies should also be thus coupled to or decoupled from the data bus.